The present invention relates to a heating system employing both a heat pump and a source of supplemental heat, such as a resistance heat furnace or a fossil fuel furnace, and relates in particular to a control system whereby greater efficiency in the utilization of the two heat sources is realized.
A heat pump utilizes a compressor and a refrigerant recirculation system including a condenser and evaporator to provide both cooling in the warmer seasons of the year and heating in the winter. When heating, the high temperature condenser is located in the interior space and the lower temperature evaporator is located outdoors to extract heat from the outside air and transfer it to the interior space through the condenser.
Since heat pumps utilize outside ambient air in contact with the evaporator as the heat source during the winter months, they operate efficiently only when the outside air temperature is above a certain level, such as 40.degree., for example. In regions of colder average winter temperatures, supplemental heat, such as is supplied by fossil fuel furnaces or resistive heat, is necessary in order to maintain the temperature within the building at the desired level. As the outside temperature drops, there is less heat available for transfer to the interior condenser, so that the system eventually reaches a point where the heat transfer is not adequate to satisfy the heat demand called for by the thermostat. Furthermore, as the outside ambient temperature drops, the efficiency of the heat pump suffers because of frost buildup on the evaporator coils, which occurs at a greater rate with a progressive decrease in the outside ambient temperature.
In the defrost cycle, the heat pump is run in the reverse direction to transfer heat from the warmer indoor condenser coil to the outside evaporator coil, thereby melting the frost. Following the defrost cycle, normal operation can be resumed, assuming that heat is called for by the thermostat. Of course, during the defrost cycle of the heat pump, heat is not being supplied to the building, and the supplemental heat must be relied on to maintain the desired ambient temperature. This requires that both the supplemental heat unit and the heat pump be operated simultaneously, the former to maintain the desired heat level within the building and the latter to eliminate the frost build-up so that the heat pump can return to normal operation.
Whenever the outside ambient is below that which permits adequate transfer of heat, both the heat pump and the supplemental heat source are operating simultaneously, with greater energy demand than with the heat pump operating alone or with the supplemental heat operating alone. When the heat pump and supplemental heat source are operating together beyond a certain portion of the heat cycle, there is greater energy consumption than if only the supplemental heat source alone is used for a given quantity of heat delivered. During the defrost cycle of the heat pump, energy is required to heat the outside evaporator coils, and supplemental heat may be necessary to maintain the desired inside temperature level called for by the thermostat. Accordingly, if frequent and lengthy defrost cycles are necessary to maintain the evaporator coils free of frost, less energy will be consumed by operating the supplemental heating alone and shutting down the heat pump entirely. This is true even though the heat pump operation is generally more efficient than supplemental heating, for example resistance or fossil fuel burning, depending on the outside temperature and humidity conditions. The buildup of frost on the evaporator coils is a function of the outside ambient temperature and also the dew point. If the dew point is high, moisture will condense on the evaporator coils and turn to frost at a higher temperature than if the dew point is lower.
As an example of the increased energy requirements for running the heat pump and supplemental heat source in tandem, consider a heating system utilizing a 15 kilowatt heat pump and a 30 kilowatt resistive heater. The heat cycle is one hour, and in this one hour period, the interior zone to be heated will demand heat input. The most efficient case exists when heat is required solely from the heat pump because maximum energy consumed is 15 kilowatt hours. However, should conditions require that the heat pump and resistive heater be run in tandem, their maximum energy consumption is 45 kilowatt hours, which is substantially less efficient than if only the resistive heater ran for the cycle consuming 30 kilowatt hours. Weather conditions can permit either the heat pump operating alone or operating in tandem with the resistive heater during the heat cycle.
Prior art control of heat pump operation is generally accomplished by means of an electromechanical thermostat mechanism, with separate temperature sensors for each state of heat pump system operation. Furthermore, there are defrost timers, relays, and pressure and temperature sensors utilized to control system defrost cycling, but since they are internal to the heat pump system, they are not readily available for immediate user control. Furthermore, the weather and condition of the heat pump system form a complex set of factors that are constantly changing, thereby making it very complex to determine the combination of heat pump and resistive heating which renders maximum efficiency. To maintain the heat pump system in the most efficient state would require the user to continually measure all of these factors and perform complex computations. Accordingly, prior art heat pump installations do not have the means available to the user to efficiently operate the heat pump systems in the lowest energy demand state while maintaining the temperature of the building at the desired level.
To summarize, the current problem with heat pump installations is that their advantage over other methods of heating exists only when the heat pump operates without supplemental heat. The more frequently that the heat pump operates with supplemental heat, either during its heating cycle when the outside ambient temperature is so low that the heat pump is not able to satisfy the heat demand, or during its defrost cycle, the less advantage there is in terms of energy efficiency over other heating plants, such as resistance or fossil fuel furnaces. This has resulted in heat pumps being used more often in regions where the outdoor temperatures are sufficiently high during the winter months that the need for supplemental heat is infrequent, such as in the Southern and Southwestern regions of North America. The use of heat pumps in cooler Northern climates, particularly in those climates where the air humidity is high during the winter months, requires very complex controls which, although perhaps they can be justified for large buildings, are not feasible for domestic and smaller commercial and industrial installations.